This invention relates to sensors and more particularly relates to active or passive sensors for sensing position shifts, motional velocities and rotational speeds of an encoder.
In the automotive industry, systems which employ wheel speed sensors for controlling brakes by means of an anti-lock system, are generally known. The devices typically include an encoder which is mechanically connected to a rotating wheel and a sensor sensing the encoder in a non-contact manner.
German published patent application No. 34 00 870 discloses a plastic sensor wherein a first housing part is a coil carrier intended for accommodating a pole core, a coil and two connecting bars. After the first housing part has been fitted with the heated electric components, it is partly spray-coated with plastics in an injection-molding operation to produce a second housing part in which is also embedded the end of a line The sensor which is generally formed of the two housing parts in addition has an opening that is closed by inserting a cover after the line has been connected to the connecting bars. A sensor of this type is exposed to great stress, especially due to moisture, dirt, heat and vibrations. The form lock with the second housing part achieved by injection-molding the first housing part and the insertion of the cover, do not reliably protect the sensor against the ingress of moisture which may cause disturbances or malfunctions of the sensor. Therefore, both housing parts, and the cover with the second housing part, are molecularly interfaced in their contact zone by ultrasonic welding. This measure increases costs incurred for the sensor because it necessitates additional process steps and a considerable expenditure in apparatus.
For this reason, a sensor as disclosed in German published patent application No. 39 30 702 was developed, wherein the seal-tightness of the housing is already provided when the second housing part is produced. When the first housing part is injection-molded by fusible plastics, a separate melting element experiences heating beyond its melting point which causes a molecular bond of both housing parts. However, the melting element which is designed as a foil, filament, or ribbon is not a component of the housing and, therefore, necessitates an additional assembly step in the manufacture of the sensor.
Further, the first housing part is interrupted at its bottom end by a pole pin which, among others, is used to fix the first housing part in position in the mold when the second housing part is injection-molded. This prevents the pole pin from being enwrapped entirely by the injection-molding plastics, with the result that leakage slots will possibly occur and allow the ingress of moisture between the pole pin and the first housing part. Further, no additional possibility of positioning the first housing part in the mold is provided in the above-mentioned prior art apart from the fixed positioning of the first housing part by the pole pin or shoe. Therefore, the first housing part and the electric components connected thereto may be displaced in the mold due to the high melting pressures occurring in the injection-molding process when the second housing part is molded.
European patent application No. 0 357 77 discloses a method and a device wherein a heat-resistant component is positioned in a mold by way of projections and spacers. These positioning elements are an integral part of the component to be spray-coated and, thus, also heat-resistant. For this reason, there is no sealing connection and, more particularly, no molecular bond between the component and the spray-coating material. This produces leakage paths which allow the entry of moisture and contaminants into the component.
An object of the present invention is to provide an active or passive plastic sensor which permits in its making an accurate and stable positioning of the electric components in the mold, which reliably prevents the ingress of moisture and, in addition, permits simple and low-cost manufacture.
According to the present invention, at least one positioning element is provided which is formed integrally with the first housing part and used to fix the first housing part in position in the mold during the manufacture of the second housing part. Displacement of the electric components is thereby avoided. To reliably prevent the ingress of moisture between preferably a plurality of positioning elements and the second housing part, the second housing part is molecularly interfaced with the positioning elements during the injection-molding process of the second housing part.
The molecular bond is facilitated by at least one melting element, preferably a plurality of melting elements though, which are integrated with the positioning elements. To prevent the ingress of moisture at different locations in the first housing part, for example, at the bottom end where the pole shoes enter in a passive sensor, the melting ribs may also be designed directly on the first housing part.
In a preferred aspect of the present invention, the melting process is particularly simple when the melting elements are configured as thin ribs which are partly liquefied when injection-molding the second housing part.
It is appropriate that the melting elements are made of a thermoplastic material which has a melting point that is lower than, or at most equal to, the melting point of the material of the second housing part. At a too low melting temperature of the ribs, the ribs would be completely molten and unable to establish a molecular bond with the second housing part. If the melting temperature of the ribs was much higher, compared to that of the second housing part, the ribs would not melt on the surface.
Especially as far as active sensors are concerned, the small overall size of these sensors, compared to passive sensors, frequently does not permit the accommodation of all electric components in a first housing part. Therefore, it is especially favorable in the injection-molding process for making the second housing part that the components can be fixed in position in the mold by preferably a plurality of additional positioning elements. The additional positioning elements, in turn, establish a molecular bond with the second housing part. This occurs particularly by way of the mentioned melting ribs provided on these elements.
According to one embodiment of the present invention, at least one portion of a positioning element is configured as a bore. The bore is favorably used to accommodate pins or pin-like elements by which the first housing part can be fixed in position in a mold. The advantage is that the positioning elements are not required to include portions which project from the sensor housing in order to fix the first housing part. Advantageously, this eliminates the need of removing these portions in the manufacture of the sensor of the present invention.
It is preferred that the positioning elements extend in a radial direction from the center line of the sensor outwardly in the direction of the mold. Twisting or dislocating of the first housing part with the electric components accommodated therein is reliably prevented by a direct form-locking connection of the positioning elements with the mold.
The melting ribs are arranged especially radially circumferentially about the axis of the positioning elements in order to shut off any possible leakage gap between the positioning elements and the second housing part.
However, the melting elements may also be arranged on any other end of the first housing part where individual electric components project from the first housing part. In turn, possibly existing leakage gaps are shut off by way of the melting ribs which are radially circumferential with respect to the center line of the sensor.
To directly fix the first housing part in position in the mold, it is preferred that the positioning elements project at least slightly from the outside contour of the sensor. The projecting portions can be embedded in the mold in a simple fashion.
In this arrangement, the positioning elements are favorably so configured that they can easily be separated, for example, cut off, after injection-molding the second housing part.
The electrically conductive connection between the contacts of the electric components and the cable conductors which extend out of the sensor can be provided, according to the present connection, by way of a molecular bond, such as soldering or welding, and by way of a form-locking, especially a mechanical, connection. Favorable combinations of these two types of connection are also possible. In particular, the electrically conductive connection of the components is effected by crimped connections according to the present invention. Thus, possibly occurring tensile forces in cables are taken up by the crimped sleeves rather than by the contacts. The crimped connections can be positioned in the first and the second housing parts.
The crimped connection can be anchored in the first housing part. In this variation of the present invention which is called xe2x80x98pre-mold Ixe2x80x99, the forces which act upon the cable conductor during molding the second housing part can be absorbed by the crimped sleeves and, thus, by the first housing part.
However, it is also possible in a variation of the present invention called xe2x80x98pre-mold IIxe2x80x99 that the crimped connections are located in the second housing part. The crimped connection may then be made more easily after injection-molding the first housing part because the electronic components are adapted to be handled together with the first housing part.
In still another aspect of the present invention, the electric components have at least in part radially outwardly projecting portions. By way of these preferably lug-shaped portions of a mounting plate of the components, the electric components and, more particularly, the mounting plate of a sensor element configured as a chip can be fixed in position in the first housing part and/or the mold in the injection-molding operation. Especially when fixing the electric components in the first housing part, it is especially preferred that the first housing part has groove-type openings into which the projecting portions can be inserted. This reliably prevents the components from dislocating during injection-molding the second housing part and permits precisely positioning the electric components in the first housing part. This permits exactly determining especially the position of the reading point of the sensor element in the sensor.
The method of the present invention used to manufacture sensors of this type is especially simple and inexpensive. Initially, the respective electric components are injection-molded with the first housing part or fixed therein by embedding or inserting. Subsequently, the first housing part including the electric components incorporated therein is fixed in position in a mold by way of the positioning elements. The second housing part is then injection-molded around the first housing part.
The materials of the positioning elements and the second housing part are preferably similar, and have at least roughly the same melting point, thereby establishing a molecular bond between these parts during injection-molding the second housing part.
In particular in active sensors which have a very small overall size, electric components which are disposed outside the first housing part can also be fixed in position in the mold by additional positioning elements in the second process step. This prevents dislocation of these components during the subsequent injection-molding process.
In a process step following the main process, portions of the positioning elements which possibly project from the outside contour of the sensor can be severed. This permits most different shapes of the positioning elements at various locations of the first housing part which do not impair the size and final shape of the respective sensor though.
The contacts of the electric components are connected in an electrically conductive manner to the cable conductors in one single step by crimped sleeves, preferably before injection-molding the second housing part. The crimped sleeves take up cable tensile forces what obviates the need for a nozzle.
In one embodiment, the crimped connection is already established before the first housing part is injection-molded. This way the crimped connection can be arranged in the first housing part, with the result that forces which act upon the cable conductors during injection-molding of the second housing part are taken up by the first housing part by way of the crimped connection.
Alternatively, the crimped connection can also be produced only after injection-molding the first housing part. It is then easier to make the crimped connection because the electronic components are easier to handle along with the first housing part.
As has been explained hereinabove, at least parts of the electric components are positioned in the first housing part and/or the mold preferably by way of portions which radially project from them. Positioning the components can be effected by injection-molding the electronic components with the first housing part or by inserting the projecting portions or lugs in grooves of the first housing part. This effectively prevents the components from dislocating during the injection-molding process and exactly predetermines the position of the components in the first housing part and, thus, in the sensor.